Since aluminum alloys are easy for cutting, they are used for shaft bearings, optical parts and automobile parts by taking advantage of this characteristic. Processability of chips (machining debris) is regarded as important in cutting the aluminum alloy, and it is desirable that the machining debris is cut into small pieces without forming long continuous stripes. However, the chips are hardly cut into pieces due to gentle collision between the chips and a cutting blade in recent years since a rake angle is provided on the cutting blade so that the surface roughness of a product is reduced in cross feed cutting.
Aluminum alloys with good cuttability that have been used conventionally include extruded materials of JIS 2011 alloy produced by adding Pb and Bi to an Al—Cu alloy and JIS 6262 alloy produced by adding Pb and Bi to an Al—Mg—Si alloy. However, aluminum alloys having good cuttability without adding Pb have been required in recent years, in light of environmental problems. Accordingly, alloys prepared by adding Sn and Bi without adding Pb has been proposed as substitutes for the JIS 2011 alloy (prepared by adding Pb and Bi), and free-cutting aluminum alloys having performance approximately equivalent to JIS 2011 alloy in cuttability (machinability) and corrosion resistance are being distributed in the market (Japanese Patent Publication No. 2726444).
However, it was a problem of these conventional free-cutting alloys that machined materials are cracked under heavy cutting conditions, such as high speed cutting. This problem is caused by embrittlement of the alloy since the machined material is heated to a temperature of as high as 135° C. or more due to the heat generated by cutting. The embrittlement of the alloy occurs at near an eutectic temperature among the added elements, and may be confirmed by measuring temperature dependency of Charpy impact test values. It may be also apprehended that the product formed by cutting may arise a brittle rupture in use at a high temperature.
The Sn—Bi-series free-cutting aluminum alloy may arise cracking in a pointing step before die drawing or in the die drawing step in the production process of the alloy as well as during cutting, to thereby induce reduction of productivity.
While these tendencies were observed in the Pb—Bi containing free-cutting aluminum alloy, they were more remarkable in the Sn—Bi containing free-cutting aluminum alloy that uses no Pb.